1) Field of the Invention
The present invention relates to a frame transmitting apparatus and a frame receiving apparatus that respectively transmits and receives frames transmitted over the synchronous digital hierarch (SDH) network or the synchronous optical network (SONET), and more particularly, to a frame transmitting apparatus and a frame receiving apparatus that efficiently control a data transmission over the SDH/SONET network without lowering efficiency of a transmission process.
2) Description of the Related Art
Sending Ethernet (registered Trademark) packets over SDH/SONET network by a technology known as Ethernet (registered Trademark) Over SDH/SONET (EOS), has been gaining ground in recent years.
FIG. 11 is a drawing for explaining a conventional EOS technology. In the conventional EOS technology, client devices 1a to 1h send packets over Ethernet (registered Trademark) in the form of a local area network (LAN) 3a and 3b. Transmitting apparatuses 2a and 2b map the packets to virtual container (VC)/synchronous-transport-signal (STS) frames of SDH/SONET and send the VC/STS frames to the opposing transmitting apparatuses 2a and 2b over an SDH/SONET network 4. The client devices 1a to 1h may be routers, and the like.
Upon receiving the SDH/SONET frames, the transmitting apparatuses 2a and 2b convert the SDH/SONET frames into Ethernet (registered Trademark) packets, and send the packets to the client devices 1a to 1h. 
The client devices 1a to 1h and the transmitting apparatuses 2a and 2b are provided with oscillators that generate clock signals of a predetermined frequency and carry out data creation and data reading based on the clock signals.
Large variations in the frequencies of the clock signals generated by the oscillators of different devices lead to erroneous data reading. Therefore, Institute of Electrical and Electronic Engineers (IEEE) has stipulated that the difference of frequencies of the clock signals between different devices shall be limited to no more than ±50 ppm.
However, even if the difference of the frequencies of the clock signals between different devices is limited to ±50 ppm, problems in effective transmit of packets may still arise if the frequency of the clock signal is far greater than the standard value.
For instance, let us assume an instance where data is being sent from the client devices 1a to 1d to the client devices 1e to 1h via the transmitting apparatuses 2a and 2b. If the frequency of the clock signals of the client devices 1a to 1d is greater than the frequency of the clock signals of the client devices 1e to 1h, the data sent by the client devices 1a to 1d slowly builds up in the transmitting apparatus 2b, resulting in a possible packet loss.
Further, if there is a limit on the flow rate of data from the client devices 1a to 1d in the SDH/SONET network 4, and if the frequency of the clock signals of the client devices 1a to 1d is large, the data sent from the client devices 1a to 1d slowly builds up in the transmitting apparatus 2a, again resulting in a possible packet loss.
As a countermeasure for packet loss, a range limit method is disclosed in Japanese Patent Laid-Open Publication No. 2002-353979. If the number of packets received from the opposing device exceeds a certain value, a PAUSE packet stipulated by the Ethernet (registered Trademark) standards is sent to the opposing device to control the flow of packets from the device.
However, in the conventional technology disclosed in the above literature, the efficiency of the transmission process carried out over the SDH/SONET network 4 is compromised.
Specifically, when a PAUSE packet is to be sent to the transmitting apparatus 2b over the SDH/SONET network, the transmitting apparatus 2b has to map the PAUSE packet on a VC frame or an STS frame. Consequently, the amount of normal data sent by the transmitting apparatus 2b gets limited, resulting in compromised transmission efficiency.
Again, there is no solution to the problem of accumulating packets sent from the client devices 1a to 1d in the transmitting apparatus 2a arising from the high frequency of the clock signals of the client devices 1a to 1d. 
Thus, effectively controlling the data transmission process without compromising the efficiency of data transmission over the SDH/SONET network 4 has become an important issue that needs tackling.